Ronald Tillery of the Commercial Appeal initially reported the deals.
McCallum has been in the league for three years, most recently with the San Antonio Spurs in 2015-16, during which he appeared in 31 games as a reserve. He averaged just 8.3 minutes per game and put up just over two points per game.
The Spurs waived him toward the end of February to make room for the newly acquired Andre Miller.
The Grizzlies are in desperate need of some depth at the guard position after the team waived Mario Chalmers due to a season-ending Achilles injury and Achilles tendinitis sidelined Mike Conley. The team announced on Saturday that Conley will be re-evaluated in three to four weeks.
Memphis also signed guard Briante Weber to a 10-day deal from the NBA D-League on Friday to help add bodies to the rotation. He played 39 minutes that night against the New Orleans Pelicans and scored 10 points with seven assists.
Injuries are what also brought Stepheson to Memphis. Center Chris Andersen experienced "shoulder subluxation" on March 6 against the Phoenix Suns, and there is no timetable for a possible return.
Stepheson has been involved with the Grizzlies before. He signed a contract in October of 2015 but was released less than a month later. He averaged 15.9 points per game for their D-League affiliate this season.
He signed a pair of 10-day contracts in February and March with the Los Angeles Clippers and appeared in four games with the team. But on Saturday morning, Brad Turner of the Los Angeles Times reported that the Clippers will not bring Stepheson back.
This isn't the best time to experience a roster shake-up of this magnitude. With 18 games remaining in the regular season, the Grizzlies are fifth in the Western Conference playoff race. They do have a decent cushion to work with, as the sixth-place Portland Trail Blazers are 4.5 games behind them.
But the team might look upon McCallum and Stepheson to help stabilize things and keep the Grizzlies afloat during the final stretch of the season.
Stats courtesy of Basketball-Reference.com